There is a strong demand for reduction in weight and thickness of a battery used as a power source of a portable apparatus, an electric vehicle, and the like. However, a package (metal can) of a conventional battery has a limitation in reducing the weight and thickness. Thus, a film is used as a package whose weight and thickness can be reduced. Specifically, a metal thin film or a laminate film formed by laminating a metal thin film and a heat-fusible resin film is used as a package of a battery. The metal thin film film and the laminate film also has the advantage in which there is a higher degree of flexibility for changing the shape than in the case of a metal can.
As a typical example of the above described laminate film, there is listed a three-layer laminate film which is formed by laminating a PP layer (polypropylene layer) as a heat fusion layer on one surface of an aluminum thin film as a metal thin film, and a nylon layer or a PET layer (polyethylene terephthalate layer) as a protection layer on the other surface of the aluminum thin film.
A common film-packaged battery is configured such that a battery element formed by laminating a positive electrode plate and a negative electrode plate via a separator is surrounded by a laminate film, and that the peripheral portion of the laminate film is hermetically heat-fused. Further, in order to lead the positive electrode and the negative electrode of the battery element to the outside of the laminate film, there are provided a positive electrode lead and a negative electrode lead, the end of one lead being connected to the positive electrode plate or the negative electrode plate, and the end of the other lead being led to the outside of the laminate film. As the separator, a porous film formed by using a thermoplastic resin, such as polyolefine, is generally used.
A configuration of a conventional film-packaged battery will be described in more detail with reference to FIG. 9. FIG. 9 is a longitudinal sectional view of a conventional film-packaged battery.
Film-packaged battery 301 has battery element 302 and a package for storing battery element 302 together with an electrolytic solution. Battery element 302 is configured by alternately laminating a plurality of positive electrode plates and a plurality of negative electrode plates via separators. Each of the positive electrode plates is formed by coating a positive electrode material on an aluminum foil, and each of the negative electrode plates is formed by coating a negative electrode material on a copper foil. Uncoated portions (extending portions) of the aluminum foil and the copper foil, on which portions the electrode materials are not coated, are led to the outside of the lamination area. The extending portions of the respective positive electrode plates (positive electrode extending portions 303a) are collectively joined to positive electrode lead 304a. Further, the extending portions of the respective negative electrode plates (negative electrode extending portions 303b) are collectively joined to negative electrode lead 304b. Note that ultrasonic welding is generally used for the joining of positive and negative electrode extending portions 303a and 303b. Further, positive electrode lead 304a and negative electrode lead 304b are produced by punching out an aluminum plate and a copper plate.
The package is configured by two sheets of laminate films 305 and 306 which surround battery element 302 by sandwiching both sides of battery element 302 in the thickness direction thereof. Each of laminate films 305 and 306 is formed by laminating PP layer 310 as a heat fusion layer, aluminum layer 311 as a metal layer, and nylon layer 312 as a protection layer.
Respective laminate films 305 and 306 surround battery element 302 in an orientation in which PP layer 310 faces the inside. The peripheral portions of facing PP layers 310 are heat-fused to each other.
In the film-packaged battery having the above described configuration, the joining portion of the positive and negative electrode extending portions, the positive and negative electrode leads, and in particular, the sharp corners of the electrode extending portions and of the electrode leads may be brought into contact with the heat fusion layer of the laminate film due to vibration, or the like, so as to damage the heat fusion layer. Further, when the heat fusion layer is damaged, the thickness of the damaged portion is reduced, so that the insulating property is deteriorated. As a result, the possibility that the metal layer as the lower layer of the heat fusion layer will be electrically short-circuited with the corner of the joining portion, is significantly increased.
In order to cope with such problem, there is proposed an insulating spacer having a triangular cross section, which houses each of the joining portions between the terminals of the positive and negative electrodes and the leads of the positive and negative electrodes (International Patent Publication No. WO 00/59063 pamphlet). The insulating spacer has an insertion hole into which each of the leads can be inserted. The insulating spacer houses an aggregate portion of the terminals of the positive and negative electrodes and fixes the laminated electrode by pressing the end surface of the laminated electrode, thereby preventing breakage of the lead, damage of the outer package film or the electric short-circuit between the outer package film and the laminated electrode.
The conventional technique including the technique as disclosed in the above described pamphlet are based on the premise that the heat fusion layer of the laminate film is damaged by the corner portion of positive and negative electrode collecting portions during use of the film-packaged battery.
However, as a result of an extensive investigation, the present inventors have found that in many cases, the heat fusion layer of the laminate film is already damaged in the manufacturing process of the film-packaged battery. Specifically, when the facing peripheral portions of the heat fusion layers are heat-fused to each other, the heat fusion layer is softened by heating. When a part of the battery element is brought into contact with the heat fusion layer in this state, the heat fusion layer is partially damaged. Since the corner of the joined positive and negative electrode extending portions is sharp in particular, when the corner is brought into contact with the softened heat fusion layer, a flaw like a deep cut is left in the heat fusion layer. FIG. 10 schematically shows the state of the heat fusion layer damaged by the corner of the joining portion of the positive and negative electrode extending portion. In the figure, reference numeral 400 denotes a heat fusion layer, reference numeral 401 denotes a joining portion of the positive electrode extending portion, reference numeral 402 denotes a metal layer, reference numeral 403 denotes a protection layer, and reference numeral 404 denotes the flaw left in heat fusion layer 400. Note that in FIG. 10, the counterpart laminate film which is heat-fused to laminate film 405 configured by heat fusion layer 400, metal layer 402, and protection layer 403, is not shown.
Here, it is obvious that the above described damage is more easily caused and the degree of the damage is larger in the case where the corner of joining portion 401 is brought into contact with heat fusion layer 400 softened by heating than in the case where the corner of joining portion 401 is brought into contact with heat fusion layer 400 during use of the film-packaged battery, at least after the heat fusion process is completed and the heat fusion layer is cured.
The above described damage in the heat fusion layer during the manufacturing process cannot be sufficiently avoided only by providing an insulating spacer as disclosed in the above described pamphlet or other protection member made of resin between the joining portion of the positive and negative electrode extending portions, and the heat fusion layer. That is, in the case where the protection member made of resin has a melting point equal to or higher than that of the heat fusion layer, when the heat fusion layer is softened so as to be able to be heat-fused, the protection member has a stiffness equal to or higher than that of the heat fusion layer. Therefore, when the protection member is brought into contact with the softened heat fusion layer, the heat fusion layer is damaged similarly to the case where the joining portion of positive and negative electrode extending portions are brought into contact with the heat fusion layer. For example, PP generally used as the heat fusion layer has a melting point of 140° C. to 150° C. Therefore, in the case where the heat fusion layer is formed of PP, when the protection member is formed of a material having a melting point of 140° C. to 150° C., or higher, the heat fusion layer is, on the contrary, damaged by the protection member.